Field of the Invention
The present invention relates to a method and apparatus for rotating a casing string in a well bore, and more particularly for rotating a segment of a casing string in a relatively highly deflected well bore.
Description of Related Art
The excess drilling mud cake which builds up on the walls of a well bore during drilling must be removed prior to cementing of the casing. It is common practice to do this by lowering the casing string into the drilled well bore until a cement shoe on the string seats upon the bottom of the bore hole. Clean drilling mud is then circulated at a relatively high rate through the casing, out of the cement shoe, and upwardly through the annulus between the casing and the well bore. Centralizers along the length of the casing string engage the well bore walls and attempt to center the casing to define a uniform annular flow area in a plane perpendicular to the direction of mud flow. This equalizes pressure distribution and flow resistance around the casing and facilitates displacement of the drilling mud. However, the clean drilling mud tends to form channels in the gelatinous, rubberlike mud cake in the annulus. If such channels were allowed to remain, the subsequently injected cement slurry would follow these channels. Eventually the mud cake between the resulting cement channels could erode away and permit communication of fluid and/or gas to the surface.
Successful removal of excess mud cake has been accomplished by mounting scratchers, turbulators and the like to the casing exterior. These frictionally engage the well bore surfaces. Suitable means are then employed to rotate the casing so that such devices brush or wipe against the mud filter cake and break the gel. The circulating clean mud can then completely scour out the well bore by carrying away the excess mud cake and loose formation materials.
After the bore hole has been circulated with clean drilling mud for a period of time, the cement slurry is introduced into the well bore annulus by passing it through the casing string and out of the cementing shoe at the bottom. Rotation of the casing string is continued during placement of the slurry to agitate the slurry and achieve uniform distribution of the slurry throughout the annulus. The cement sheath which results is absent any continuous channels of drilling mud which might erode away during well treatment and production. U.S. Pat. No. 3,828,852, issued Aug. 13, 1974 to C. G. Delano is illustrative of the foregoing method.
The foregoing system for rotating a casing string is not possible in a relatively high angle or highly deflected well bore. If a sufficient number of centralizers were used to center the casing throughout its length, the friction between the centralizers and the bore hole walls would ultimately prevent insertion of the casing. Without such centralizers the casing sags into engagement with the well bore walls at high angle bends and frictional forces then exist which are so great that the casing string cannot be rotated. This results in an inferior placement of cement and expensive and time consuming squeeze procedures must be adopted. Many wells are slant drilled, particularly in offshore waters, and the establishment of a cement sheath or seal of high integrity is critical to prevent catastrophic pressure releases.